During playback of moving images—that is, the sequential presentation at appropriate points in time of a series of still images (frames)—a viewer may sometimes observe an undesired brightness variation, ‘flicker’, which was not present in the depicted scene. Flicker may be caused by a light source having intensity oscillations that are fast enough to be imperceptible to the human eye. However, the recording includes sampling, at the frame rate of the imaging apparatus, of this oscillation frequency, which may give rise to a lower, visibly perceptible frequency through the process of sampling. FIG. 1 illustrates how samples (shown as circles) of a high-frequency signal can be interpreted as coming from a low-frequency signal and vice versa; this phenomenon is referred to as aliasing.
One may distinguish different kinds of flicker. In a gray-scale video sequence, flicker is an unintentional—and usually periodic—variation of the single channel of the image signal. Such variation may affect the whole frame or only a sub-region, which may correspond to a region of space having a particular illumination. When color video technology is used, an oscillating white light source may affect the recorded video sequence in a different way than an oscillating colored light source. As will be explained in the next few paragraphs, the precise interpretation of flicker in terms of image components depends on the precise color video format used.
Firstly, if the video sequence is encoded in terms of linear primary color components, such as RGB, the undesired oscillation will be present in all components in the case of a white light source. If the oscillating light source is colored, it will contribute an oscillating term to each color component in proportion to the composition of the color of the light source; for example, an oscillating red light source will contribute predominantly to the R component of an RGB signal and less to the G and B components.
Secondly, several widespread color video formats are based on a three-dimensional YCbCr color space. Such a video format comprises one luma channel Y (encoding the luminance component, or brightness, of a pixel) and two chroma channels Cb, Cr (encoding the chrominance components of a pixel in terms of the deviation from white). The precise definition of the image components (as regards constants, scaling, offset etc.) may vary between different particular video formats, but generally there exists an unambiguous transformation (sometimes a linear transformation) between a primary color format and a YCbCr format.
Thirdly, there exist further color video formats based on the triple of hue, saturation and lightness, notably the HSL, HSV, HLS, HIS and HSB formats. Generally, a transformation to and from the RGB format accompanies each video format of this kind. Flicker, at least white flicker, will be detectable in the lightness/value/brightness/intensity channel (L or V), which will not be distinguished from luminance in the rest of this disclosure.
The discussion in the previous paragraphs intentionally does not distinguish between analogue and digital formats since, for the purposes of this disclosure, the latter may be regarded as quantized versions of the former. Likewise, some video formats may exist in gamma-compressed or partially gamma-compressed version, such as the R′G′B′ and the Y′CbCr formats, in addition to the linear version. However, it is immaterial for the understanding of the present invention whether the video format includes such compression.
Because viewers may find flicker disturbing or unpleasant, there has been an interest in the field of video processing in correcting it. Several available methods for suppressing or removing flicker are based on correction of each frame in a flickering sequence against a reference frame. More precisely, a cumulative distribution function (CDF) or, by another name, a cumulative histogram is generated for the frame to be corrected and a reference CDF is generated for the reference frame. The pixel values are then adjusted in order that the CDF for the corrected frame is approximately equal to that of the reference frame. In general, it is not necessary for the reference frame to be identical (apart from the brightening or darkening caused by flicker) to the frame to be corrected, but it should preferably depict a similar scene with respect to background, lighting, etc. Furthermore, there are known methods which use histograms rather than cumulative histograms.
Flicker correction by (cumulative) histogram methods often leaves artifacts that are generally most noticeable in bright spatial regions of the image. Frames at one stage of the flicker cycle may resolve image features that are not resolved in frames in another stage of the flicker cycle. Such residual flicker is a severe disadvantage of conventional methods based on cumulative histograms. The same effect also occurs in conventional histogram-based methods.